Erosion is a natural process caused by the lack of vegetation resulting from wildfires, construction projects or a multitude of other factors. The amount of erosion after a burn depends on storm events, burn severity, slope, and soil type. Erosion may be immediate and can continue over several years as root systems of burnt vegetation decay.
Debris flows and mudslides are extreme forms of erosion and can deliver huge amounts of sediment and debris downslope which can result in extensive property damage and in worst cases, loss of life.
If you live on or near a slope, in a draw or ravine, or along a stream or river, you need to be aware of the extent of wildfire damage upslope or upstream. If most or all of the vegetation has been burned off or removed of these areas, the chance for erosion is significantly greater and can result in mass movements of soil and water downhill or downstream from the damaged areas. To be safe, assume all drainages in steep hilly areas can carry debris flows and that they are vulnerable after a wildfire.
Surface erosion is when individual soil particles move, usually by water flowing over exposed soil. Surface erosion is reduced by several processes. The first is interception of rainfall by foliage which reduces the velocity and impact of the droplets. The second is retention- as roots anchor soil particles and reduce the likelihood of their migration. The third is retardation - the presence of vegetation slows the movement of runoff across a surface, decreasing soil movement. Finally, infiltration - the roots of plants open up many paths of entry into soil thereby increasing opportunities for water to soak in rather than runoff.
At Elliott Land Management, we have a number of approaches to ensure the Native plants take root. Whether it is a simple irrigation plan, or a more extensive bio-engineered structure.
The most stable slopes are promoted by vegetation. Roots provide reinforcement and buttressing, particularly when there is bedrock that they can penetrate within the rooting zone. Additionally, transpiring plants remove excess water from soil, reducing the weight and improving the strength of soil. Plantings can be reinforced with other natural materials in the first few years while they get established.
Barriers are installed on hill slopes and in streams to slow water flow, increase infiltration, and trap sediment. Types of barriers include log barriers, straw wattles, sandbags, silt fences, and straw bale check dams. Barriers are rarely used in our area due to ecological and flooding concerns but the idea behind them is useful when considering the approach to an issue you may have.
Seeding and Revegetation
Grass seeding is the most commonly used erosion control treatment after a burn. Grass is seeded to burned sites from the ground or by air to increase vegetative cover during the first few critical years after a fire.
The number of variables contributing to erosion along with the costs for cleaning up the effects can be extensive. No matter which way you look at the effects of soil erosion, the best remedy always comes back to keeping the soil where it belongs.